/Engine/Source/Runtime/Portal/Services/Public/Account/IPortalUserLogin.h
If a user is already signed into the Portal then a prompt may ask the existing user if they want to sign-out, especially if they have on going installations or downloads. The user may opt to not sign-out if this happens.
whether the portal successfully received and handled your request. Note this doesn't mean that the user signed-in successfully or even that the existing user signed-out. It simply means that the Portal attempted to prompt the user for sign-in.
[TAsyncResult](API\Runtime\Core\Async\TAsyncResult)< bool > PromptUserForSignIn()